If something is the sightliest, it is the most pleasant or attractive to look at.
The word sightliest is used to talk about how things look. It is usually a very good thing.
Examples:
The flower in the garden is the sightliest.
The sightliest house on the street is painted blue.
She is the sightliest person in the room.
The sightliest cat in the world has a big smile.
The sightliest rainbow I ever saw had ten colors!
The sightliest view from the mountain was amazing.